Composition of nylon powder and silica aerogel



United States Pateht O CQMPGSITIGN OF NYLON POWDER AND SILICA AEROGELLaurence R. B. Hervey, West Concord, Mass, assignor, by mesneassignments, to The Polymer Corporation, Reading, Pin, a corporation ofPennsylvania No Drawing. Application May 29, 1953, Serial No. 358,559

5 Claims. (Cl. 260-37) This invention relates to the process ofproducing molded synthetic linear polyamides, and in particular toimproving the pourability of granulated synthetic linear polyamidepowder. In an application entitled Method of Preparing Finely DividedNylon Powder, Serial No. 202,405, filed December 22, 1950, in the namesof Louis L. Stott and Laurence R. B. Hervey, now U. S. Patent 2,639,278,a method of preparing finely divided synthetic linear polyamides wasdescribed. Another method was also described in an application entitledMethod of Preparing Finely Divided Polyarnides, Serial No. 273,566,filed February 26, 1952, in the names of Louis L. Stott and Laurence R.B. Hervey, now U. S. Patent 2,742,440. This latter application alsodefined the linear polyamides to which the process applies.

These polyamides as described above, or as otherwise identifiedhereafter, can be obtained, for example, by selfpolymerization ofmouoaminomonocarboxylic acid, or by reacting a diarnine with a dibasiccarboxylic acid in substantially equimolecular amounts, it beingunderstood that reference herein to amino acids, diamines and dibasiccarboxylic acids is intended to include the equivalent amide-formingderivatives of these reactants.

These linear polyamides include also polymers obtained by admixture ofother linear polymer-forming reactants, as for instance glycol-dibasicacid mixtures in the case of polyester amides, with the mentionedpolyamide-forming reactants. The best results in the practice of theinvention described herein, however, are obtained with unmodifiedstraight polyamides. In the simple polyarnides the average number ofcarbon atoms separating the amide group is at least two. On hydrolysiswith hydrochloric acid, the amino acid polymers yield the amino acidhydrochloride, and the diamine-dibasic acid polymers yield the diaminehydrochloride and the dibasic carboxylic acid. For the sake ofsimplicity, the linear polyamides described above will be referred toherein as nylon, but only those polyamides which are insoluble in thesolvents described and claimed herein at room temperature are intendedto be included in the appended claims, and are the subject of thisinvention. The polyamides to which this invention pertains, as describedabove, also may be defined briefly as fiber-forming synthetic linearpolycarbonamides in which the carbonarnide groups are an integral partof the main chain.

In general, finely divided polyamides to which this invention appliesare prepared by dissolving the polyamide in a solvent at elevatedtemperatures, the solvent being one which dissolves the polyamide atelevated temperatures but is a non-solvent at lower temperatures. Theresulting powder may be washed, if desired, to remove the solvent usedto dissolve it. The powder is dried and granulated to the proper size ofaggregate, which preferably is between 15() mesh. The ultimate particlesize of the polyamide particles within the aggregate resulting from thisprocess is quite small, being less than 40 microns in diameter. Thesteps of dissolving the precipitate and washing should be carried outunder conditions 2 ,811,499 Patented Oct. 29, 1957 described and claimedin U. S. Patent 2,695,425 in the name of Louis L. Stott.

A silica aerogel may be added to nylon powder to which one or more of anumber of fillers have been admixed. As disclosed in U. S. Patent2,695,425 in the name of Louis L. Stott up to 85 of the total bulkvolume may be filler.

The fillers that may be added cover a wide variety of materials. Ingeneral it has been found that for most applications the fillers areadvantageously finely divided, although bonding may be secured to largerpieces, if desired. If an abrasive is to be manufactured, abrasiveparticles are incorporated with the nylon; these include the usualabrasives such as diamond dust, ground silica, Carborundum, chalk,Alundum, tungsten carbide, etc. For other purposes metal powders, suchas copper, lead, or iron, may be added to give the desired propertiesand these are readily incorporated with the nylon. For still otherpurposes carbon or graphite particles, or molybdenum disulphideparticles, may be incorporated with nylon. In some cases ceramicmaterials with special dielectric properties are advantageously mixedwith nylon. Among these are titanium dioxide and various titanates, inparticular combinations of titanium dioxide with alkaline earths. Onoccasion polytetrafluoroethylene is a useful filler. The physical orelectrical characteristics of molded nylon articles may therefore betailored to meet the desired specifications where the incorporation ofother materials is advantageous. The filler should be substantiallynon-reactive with the nylon, have a higher melting point than thetemperature to which the nylon must be raised to sinter it and notsubject to any substantial amount of decomposition during the heatingstep.

In most cases, the fillers serve to reduce hydroscopic or thermalexpansion as compared to articles made entirely of nylon and aidmaterially in securing dimensional accuracy through reduced distortionduring sintering. The reasons for these improved characteristics are notclear but have been repeatedly observed.

Difliculty has been experienced, however, in pouring the granulatedpowder into the mold. This difliculty is accentuated and is particularlyacute with automatic presses where a free-flowing powder is desirable topermit adequate and rapid filling of the mold.

It is an object of this invention to improve the pourability ofgranulated nylon powder. Another object is to improve the pourability ofgranulated nylon powder Without adversely affecting the characteristicuseful qualities of the pressed articles.

These and other objects may be accomplished by adding from /2%5% byweight of a finely divided silica. A silica aerogel of the typedescribed in U. S. Patent 2,093,454 to Samuel Kistler, is particularlyeffective. A satisfactory commercial product of this type is thematerial known under the trade name Santocel.

The washed and dried granulated powder is admixed with the finelydivided silica referred to above by tumbling the two together until evendistribution of the silica is accomplished. While tumbling the twomaterials together is preferred, any other suitable means of forming anintimate mixture as by stirring them together or by any of the acceptedmeans of agitating the mixture of powders is acceptable. 7

It is surprising that finely divided silica aerogel should be unique inits property of markedly increasing the pour- Example I 150 grams of thepolymer of hexamethylene adipamide in flake form was dissolved in 850grams of ethylene glycol in a nitrogen atmosphere at 185 C. during aperiod of one hour. The solution was filtered, and cooled to 170 C. 2000cc. of distilled water at room temperature were poured into the glycolsolution with agitation. T he nylon precipitated and the precipitate waswashed until glycol-free with distilled water. The glycol-free powderwas air dried, to remove most of the water, followed by vacuum drying toreduce the moisture content to less than one percent.

The vacuum dried powder was granulated by passing it through a 16 meshscreen. 50 grams of powder were placed in a standard funnel as describedin A. S. T. M. designation 13392-38, and time of flow from the funnelwas found to be 25 seconds. To 50 grams of powder were added 0.5 gram ofSantocel with tumbling. The powder was placed in a standard funnel andtime of flow was found to be 19 seconds.

Example [I 150 grams of the polymer of epsilon caprolactam in flake formwas dissolved in 850 grams of ethylene glycol in a nitrogen atmosphereat 185 C. during a period of one hour. The solution was filtered, andcooled to 170 C. 2000 cc. of distilled water at room temperature werepoured into the glycol solution with agitation. The nylon precipitatewas Washed until glycol-free with distilled water. The glycol-freepowder was air dried, to remove most of the water, followed by vacuumdrying to reduce the moisture content to less than one percent.

The vacuum dried powder was granulated by passing it through a 16 meshscreen. A comparison was made of the flowabiiity of powder containingthe addition of 1 part by weight of aerogel to 100 parts of powder withpowder containing no aerogel. The standard A. S. T. M. method describedin Example I was followed, and the results showed that the addition ofaerogel reduced the time required by the powder to pass through thefunnel by approximately 25 percent.

Example III 150 grams of the polymer of hexamethylene sebacamide inflake form was dissolved in 850 grams of ethylene glycol in a nitrogenatmosphere at 185 C. during a period of one hour. The solution wasfiltered and cooled to 170 C. 2000 cc. of distilled water at roomtemperature were poured into the glycol solution with agitation. Thenylon precipitated and was washed until glycol-free with distilledwater. The glycol-free powder was air dried, to remove most of thewater, followed by vacuum drying to reduce the moisture content to lessthan one percent. The vacuum dried powder was granulated by passing itthrough a 16 mesh screen.

A comparison was made of the flowability of powder containing theaddition of 1 part by weight of aerogel to 100 parts of powder withpowder containing no aerogel.

4 The standard A. S. T. M. method described in Example I was followed,and the results showed that the addition of aerogel reduced the timerequired by the powder to pass through the funnel by approximately 25percent.

Example IV The procedure of Example I was followed except that 0.5% byweight of powdered polyethylene oxide having a molecular weight of 6000was intimately mixed with the nylon powder as a mold lubricant. 0.5% offinely divided silica aerogel was added by tumbling as an aid topourability. In accordance with the standard A. S. T. M. methoddesignated D39238, Testing Molding Powders Used in Manufacturing MoldedElectrical Insulators, powder without the aerogel took 21.6 seconds andpowder with the aerogel took 19.9 seconds to flow through the funnel.

Example V A mixture of 50% powdered nylon and 50% finely dividedpowdered copper would not flow under the standard test conditions. Thenylon powder was prepared, and dried in accordance with the procedure ofExample I. With the addition of 1%, 2%, and 3% silica aerogel theflowability test took 156, 242, and 336 seconds respectively.

It will be seen from the above examples that finely divided silica iseffective for increasing the flowability or pourability ofpolyhexamethylene sebacamide. Other nylon polyamides, when subdivided inaccordance with the applications referred to above, show similarincreased pourability rates upon the addition of silica in the amountsof from /25% by weight. While amounts in excess of 5% by weight do notdecrease the volume of powder per second poured into a mold, additionsof silica in excess of 5% increase the bulk of the mixture to such anextent that the actual weight of polymer which can be poured into a moldin a given time falls off to uneconomical levels if the amount of silicais increased beyond 5%.

I claim:

1. An intimate mixture comprising finely divided syntheticlinear'polyamide and a silica aerogel said aerogel being present in anamount equal to from .5% to 5% by weight of said polyamide, saidpolyamide being one which is a fiber-forming synthetic linearpolycarbonamide in which the carbonamide groups are an integral part ofthe main chain, the ultimate particle size of said polyamide being lessthan 40 microns in diameter.

2. The product in accordance with claim 1 wherein the polyamide ispolyhexarnethylene adipamide.

3. The product in accordance with claim I wherein the polyamide ispolyhexamethylene sebacamide.

4. The product in accordance with claim 1 wherein the polyamide is thepolymer of epsilon caprolactam.

5. An intimate mixture comprising a mixture of finely divided syntheticlinear polyamide, a filler and a silica aerogel, said aerogel beingpresent in an amount equal to from 5% to 5% by weight of said polyamide,the filler being present in an amount up to by volume of said mixture,said polyamide being one which is a fiber-forming synthetic linearpolyearbonamide in which the carbonamide groups are an integral part ofthe main chain, said polyamide having an ultimate particle size of lessthan 40 microns in diameter.

References Cited in the file of this patent FOREIGN PATENTS 1,005,744France Jan. 2, 1952

1. AN INTIMATE MIXTURE COMPRISING FINELY DIVIDED SYNTHETIC LINEARPOLYAMIDE AND A SILICA AEROGEL SAID AEROGEL BEING PRESENT IN AN AMOUNTEQUAL TO FROM .5% TO 5% BY WEIGHT OF SAID POLYAMIDE, SAID POLYAMIDEBEING ONE WHICH IS A FIBER-FORMING SYNTHETIC LINEAR POLYCARBONAMIDE INWHICH THE CARBONAMIDE GROUPS ARE AN INTEGRAL PART OF THE MAIN CHAIN, THEULTIMATE PARTICLE SIZE OF SAID POLYAMIDE BEING LESS THAN 40 MICRONS INDIAMETER.